1. Field of the Invention
The present invention relates to a high-temperature superconducting (hereinafter, referred to as HTS) magnetic sensor that includes a plurality of superconducting quantum interference devices (hereinafter, referred to as SQUID) on a superconducting layer formed on a substrate, and a fabrication method of the HTS magnetic sensor.
2. Description of Related Art
The SQUID is known as a magnetic sensor that has a highest sensitivity for a magnetic signal. Therefore, the SQUID is used for a measurement of a weak biomagnetic signal which is spontaneously generated from a brain or a heart and a nondestructive inspection by a remaining magnetism and an eddy-current testing, and further used for a super-low field MRI (magnetic resonance imaging) in recent years. However, an application of the SQUID has been limited, because expensive and troublesome liquid helium is required for cooling a low-temperature superconductor (hereinafter, referred to as LTS) SQUID. After a high-temperature superconductor (hereinafter, referred to as HTS) that shows a superconducting property at a liquid nitrogen temperature was discovered, a HTS SQUID has been actively developed. Then, at present, a HTS magnetic sensor having a detection sensitivity of less than 100 fT/Hz is available in the market, and further, a HTS magnetic sensor having a detection sensitivity of less than 10 fT/Hz has been reported at a leading-edge research. Since the HTS SQUID can be cooled by liquid nitrogen which is less expensive and easy to handle, the SQUID having various kinds of structures has been proposed (see the following Non-patent documents 1 to 6).
Non-patent document 1 “Chapter 1 Introduction, “The SQUID Handbook: Fundamentals and Technology of SQUIDs and SQUID Systems, Volume I”, Edited by John Clarke, Alex I. Braginski, ISBN: 978-3-527-40229-8”;
Non-patent document 2 “J. M. Jaycox and M. B. Ketchen, IEEE Trans Magn. MAG-17, 400-403 (1981)”;
Non-patent document 3 “A. Tsukamoto, IEEE Trans. Appl. Supercond. Vol. 15 No. 2 (2005) 177-180”;
Non-patent document 4 “H. Wakana, S. Adachi, K. Hata, T. Hato, Y. Tarutani, K. Tanabe, IEEE Trans. Appl. Supercond. Vol. 19 No. 3 (2009) 782-785”;
Non-patent document 5 “S. Adachi, K. Hata, T. Hato, Y. Tarutani, K. Tanabe, Physica C, vol. 468, No. 15-20, pp. 1936-1941, (2008)”; and
Non-patent document 6 “E. Dantsker, S. Tanaka, J. Clarke, Appl. Phys. Lett. Vol. 70 No. 15 (1997) 2037-2039”
A basic structure of a SQUID is proposed in the Non-patent document 1 “Chapter 1 Introduction, “The SQUID Handbook: Fundamentals and Technology of SQUIDs and SQUID Systems, Volume I”, Edited by John Clarke, Alex I. Braginski, ISBN: 978-3-527-40229-8”. The SQUID has a closed loop structure consisting of a SQUID inductor and two Josephson junctions. If a bias current Ib, which is a little larger than a critical current Ic, is applied between electrodes of the SQUID in order to generate a voltage V in the two Josephson junctions, a voltage generated between the terminals of the respective Josephson junctions periodically varies with a cycle of a flux quantum Φ0 (2.07×10−15Wb) by a magnetic flux Φ that interlinks with the closed loop structure. An extremely small magnetic flux change of 1/105 to 1/106 of the flux quantum Φ0 can be measured by a feedback control using a flux locked loop circuit (FLL circuit).
Since a size of a SQUID is small ranging from dozens of μm to hundreds of μm in general, when the SQUID is used as a high sensitive magnetic sensor, a pickup coil having a larger size is used. The pickup coil configures a flux transformer (closed loop structure) together with an input coil, and the flux transformer is used in such a manner that the input coil is magnetically coupled with the SQUID. If the effect of inductance of a wiring portion between the input coil and the pickup coil is neglected, a magnetic flux Φs detected by the SQUID when an external magnetic field B is applied to the pickup coil is expressed by Formula (1).
                                                                        Φ                s                            =                            ⁢                                                I                  p                                ·                M                                                                                        =                            ⁢                                                BA                  p                                ·                                  M                  /                                      (                                                                  L                        p                                            +                                              L                        i                                                              )                                                                                                                          =                            ⁢                                                BA                  p                                ·                                                                            k                      ⁡                                              (                                                                              L                            i                                                    ·                                                      L                            s                                                                          )                                                                                    1                      /                      2                                                        /                                      (                                                                  L                        p                                            +                                              L                        i                                                              )                                                                                                          (        1        )            where, Ls is an inductance of a SQUID inductor, Ap is an area of a pickup coil, Lp is an inductance of the pickup coil, Li is an inductance of an input coil, and M is a mutual inductance between the SQUID and the input coil.
Here, Ip is a shielding current flowing in a flux transformer, the mutual inductance M has a relation of M=k(Li·Ls)1/2, and k is a coupling coefficient (0<k<1). A ratio of the magnetic flux Φs detected by the SQUID to the external magnetic field B is called an effective area Aeff, and has a relation expressed by Formula (2).
                                                                        A                eff                            =                            ⁢                                                Φ                  s                                /                B                                                                                        =                            ⁢                                                A                  p                                ·                                  M                  /                                      (                                                                  L                        p                                            +                                              L                        i                                                              )                                                                                                                          =                            ⁢                                                A                  p                                ·                                                                            k                      ⁡                                              (                                                                              L                            i                                                    ·                                                      L                            s                                                                          )                                                                                    1                      /                      2                                                        /                                      (                                                                  L                                                                                                                                  ⁢                          p                                                                    +                                              L                        i                                                              )                                                                                                          (        2        )            
Formula (2) indicates that a detection sensitivity (magnetic flux Φs detected by SQUID) of a SQUID magnetic sensor becomes higher as the effective area Aeff becomes larger. If the inductance Ls of the SQUID is too large, a modulation voltage amplitude ΔV of the SQUID decreases and magnetic flux noises increase. Therefore, a value of the inductance Ls is generally about 40 to 100 pH. In addition, since a size and a shape of the pickup coil are determined depending on the application, values of the area Ap and inductance Lp of the pickup coil are given parameters. Therefore, adjustable parameters for maximizing the effective area Aeff in Formula (2) are the coupling coefficient k and the inductance Li of the input coil. The coupling coefficient k has the maximum value “1” when a magnetic coupling between the input coil and the SQUID is perfect, and at this time, the effective area Aeff is also maximized. On the other hand, with respect to the inductance Li of the input coil, when the inductance Li is equal to the inductance Lp of the pickup coil (Li=Lp), the effective area Aeff becomes maximum (Aeff=Ap(Ls/Li)1/2/2).
An integrated SQUID having an ideal structure is proposed in Non-patent document 2 “J. M. Jaycox and M. B. Ketchen, IEEE Trans Magn. MAG-17, 400-403 (1981)”. The integrated SQUID has a structure where an input coil having a multi-turn structure is stacked on a washer-type SQUID inductor through a thin insulating layer, and the coupling coefficient k close to “1” can be obtained. In addition, the number of turns of the input coil is optimized so that the inductance Lp of the pickup coil becomes equal to the inductance Li of the input coil (Li=Lp). However, the Non-patent document 2 describes the case of a low-temperature superconducting SQUID (LTS-SQUID) which uses Nb as the superconductor, and a fabrication yield of the integrated SQUID using a HTS is low. This is caused by the following reasons. In a HTS composed of multielement composite oxide, it is likely to generate precipitates by composition shift and segregation, and as a result, it is likely to cause a breakdown of an interlayer insulating layer between two HTS layers (top and bottom), and likely to cause degradations of thin film characteristics and a junction during a multilayer process.
In the Non-patent document 3 “A. Tsukamoto, IEEE Trans. Appl. Supercond. Vol. 15 No. 2 (2005) 177-180”, a directly-coupled SQUID is proposed. The directly-coupled SQUID can be fabricated using a single layer superconductor film. A grain boundary junction such as a bicrystal junction or a step-edge junction that may be formed using a single layer superconductor film, is used for a Josephson junction. In the directly-coupled SQUID, a pickup coil (inductance Lp) is directly connected to a slit-hole type SQUID inductor (inductance Ls), and an input coil (inductance Li) is omitted because the SQUID inductor (inductance Ls) has a function as the input coil. Namely, the directly-coupled SQUID has a structure where a shielding current Ip to be induced by a magnetic flux that interlinks with the pickup coil (inductance Lp) directly flows in the SQUID inductor (inductance Ls). In the directly-coupled SQUID, the inductance Li of the input coil is equal to the inductance Ls of the SQUID inductor since the SQUID inductor has a function as the input coil (Li=Ls). The inductance Lp of the pickup coil ranges from several nH to several tens of nH, then the inductance Lp is larger than the inductance Ls of the SQUID inductor (Lp>>Ls). The effective area Aeff of this case is expressed by an approximation formula of Formula (3) from Formula (2). The effective area Aeff of this case (coupling efficiency between the pickup coil and the SQUID) is smaller than the effective area Aeff (Aeff=Ap (Ls/Li)1/2/2) of the Non-patent document 2, as shown by the following equation.Aeff=Ap·Ls/(Lp+Ls)  (3)
In addition, the Non-patent document 3 describes that junction characteristics of the HTS also scatter in the fabrication of the SQUID using even in a single layer superconductor thin film. For example, although a critical current of SQUID about 10 to 100 μA, ideally, 10 to 20 μA, is required at the operation temperature, the critical current may be over 100 μA in some cases and the yield is decreased. Therefore, the following was attempted to improve the yield. A plurality of SQUIDs are connected in series to the same pickup coil, and a SQUID having good characteristics is selected from the plurality of SQUIDs and used in order to improve the yield. In the Non-patent document 3, an inductance Ls of SQUID inductor of the non-selected SQUID also functions as to be added to the inductance Lp of the pickup coil. Namely, the effective area Aeff in the case that n SQUIDs (nth inductance is denoted by Ls,n) are arranged in series is expressed by Formula (4).AeffAp·Ls/(Lp+ΣLs,n)  (4)
Here, ΣLs,n indicates a total sum (Ls,1+Ls,2+ . . . +Ls,n) of SQUID inductors of the n SQUIDs. Comparing with Formula (3), the denomination increases by about ΣLs,n·(n−1)/n, then, the Aeff decreases. However, in the case of the directly-coupled, generally, the inductance Lp of the pickup coil is two orders of magnitude larger than the inductance Ls of the SQUID inductor (Lp>>Ls). In this case, a decrease of the effective area Aeff due to arrangement of the plurality of SQUIDs in series is small, and it is no matter for practical use.
The research institute to which the inventors belong is promoting a development of an integrated SQUID which has a high sensitivity comparable to the sensitivity of LTS-SQUID, by using a HTS. A prototype integrated SQUID having a structure that an input coil is stacked on a SQUID inductor was fabricated successfully using a HTS multilayer process which includes two superconducting layers (See Non-patent document 4 “H. Wakana, S. Adachi, K. Hata, T. Hato, Y. Tarutani, K. Tanabe, IEEE Trans. Appl. Supercond. Vol. 19 No. 3 (2009) 782-785”, and Non-patent document 5 “S. Adachi, K. Hata, T. Hato, Y. Tarutani, K. Tanabe, Physica C, vol. 468, No. 15-20, pp. 1936-1941, (2008)”). This integrated SQUID also has a structure that an input coil having a multi-turn structure is stacked on a washer-type SQUID inductor through a thin insulating layer, and the coupling coefficient k close to “1” can be obtained. In addition, an excellent noise characteristic of 20 to 40 fT/Hz1/2 has been obtained. However, the yield was decreased in some cases, for example, by defects in interlayer insulation, in addition to variations of junction characteristics specific to a HTS.
As with the directly-coupled SQUID of the Non-patent document 3, it is considered that the yield of the HTS magnetic sensor may be improved by connecting the integrated SQUIDs of the Non-patent documents 4 and 5 in series. However, different from the case of the Non-patent document 3, it is considered that the effective area Aeff, that is, the detection sensitivity may be decreased. The effective area Aeff of a structure that arranges n integrated SQUIDs in series is expressed by Formula (5), which is derived from Formula (2).Aeff=Ap·k(Li·Ls)1/2/(Lp+ΣLi,n)  (5)
Here, ΣLi,n indicates a total sum (Li,1+Li,2+ . . . +Li,n) of inductances of input coils of the n integrated SQUIDs. In the integrated SQUID, since the inductance Lp of the pickup coil and the inductance Li of the input coil are designed to be equal (Li=Lp), the effective area Aeff is about 2/(n+1) times in comparison with the case of a single integrated SQUID. Then, regarding the integrated SQUID, it has been considered that a structure that connects a plurality of SQUIDs in series appears to be impracticable, and accordingly, there is no example of trial fabrication of the structure until now. As described above, a method for improving the yield of the integrated SQUID, which easily loses the yield, has been expected. However, if a good integrated SQUID can be selected and used from a plurality of integrated SQUIDs in a HTS magnetic sensor, there is an advantage because a yield of the HTS magnetic sensor can be improved even if the yield of individual integrated SQUID is low.
On the other hand, although the integrated SQUID is advantageous in the detection sensitivity (effective area Aeff), there is a disadvantage that a magnetic flux trap is likely to occur in comparison with a directly-coupled SQUID. A cooling of the HTS SQUID is easier than that of a LTS-SQUID which requires liquid helium for the cooling, and a wide application of the HTS SQUID is actively studied. For example, an application for outdoor usage such as a geological survey, where the procurement of liquid helium is difficult, and an application for non-destructive inspection at a factory are studied. In both cases, since the SQUID is used in an environment provided with no magnetic shield in most cases, the HTS SQUID is cooled in the Earth field. In this case, if a phenomenon to trap a magnetic flux in the superconductor occurs, noises are generated in the SQUID by the movement of the trapped magnetic flux, and as a result, a measurement of a weak signal becomes difficult. There is a relation expressed by the following Formula (6) between an intensity of a threshold magnetic field B1 at which the trapping phenomenon starts and a size of the superconductor (See Non-patent document 6 “E. Dantsker, S. Tanaka, J. Clarke, Appl. Phys. Lett. Vol. 70 No. 15 (1997) 2037-2039”). Meanwhile, w is a width of the superconductor.B1=πΦ0/4w2  (6)
A width of a SQUID inductor of a directly-coupled SQUID is about 5 μm, then, the threshold magnetic field B1 derived from Formula (6) is about 65 μT. Since the Earth field in Japan is about 30 to 50 μT, the magnetic flux trap of the directly-coupled SQUID is not likely to occur by the cooling in the Earth field. On the other hand, the integrated SQUID has a structure that stacks an input coil on a SQUID inductor in order to efficiently magnetically couple the multi-turn input coil with the SQUID inductor. As a result, the width of the SQUID inductor becomes wide, and generally, the width is about 100 to 300 μm. Even if the width is 100 μm, the threshold magnetic field B1 is 0.16 μT, which is below the Earth field. Therefore, with respect to the integrated SQUID, the magnetic flux trap is likely to occur if the SQUID is used by the cooling in the Earth field.
The Non-patent document 6 proposes an integrated SQUID that can prevent the magnetic flux trap. In the integrated SQUID, even if a width of the SQUID inductor is wide, an occurrence of the magnetic flux trap is prevented by narrowing a minimum line width that configures the SQUID inductor by, for example, forming a body of the SQUID inductor into a mesh structure, or disposing slit holes.
In addition, a dynamic range is another disadvantage of the integrated SQUID. In the integrated SQUID, a superconductive contact between two superconductor layers (top and bottom) is inevitably required at a portion of the input coil to be stacked and having a multi-turn structure. It is known that a critical current density of the HTS decreases at the superconductive contact. Therefore, the critical current of a magnetic transformer consisting of a pickup coil and an input coil is limited by the critical current at a superconductive contact of the input coil portion. This means that even if the pickup coil receives a large magnetic field change, there is an upper limit with respect to a flowing shielding current, and thereby, a dynamic range is limited. On the other hand, since a directly-coupled SQUID is formed by a single superconductor thin film, there is no superconductive contact between the two superconductor layers (top and bottom). Therefore, a dynamic range of measurement of the directly-coupled SQUID becomes wide.
As described above, an integrated SQUID is advantageous in sensitivity, and a directly-coupled SQUID has advantages that the magnetic flux trap is not likely to occur and the dynamic range is wider. Therefore, it is required to select the integrated SQUID or the directly-coupled SQUID depending on the application of the SQUID. Meanwhile, if it is capable to select an integrated SQUID or a directly-coupled SQUID in a single HTS magnetic sensor, a high sensitivity and wide dynamic range are both obtained. This is advantageous and useful.
In addition, as a cooling method of a HTS magnetic sensor, there are two methods that are a direct cooling by liquid nitrogen and an indirect cooling by thermal conduction via a rod cooled by a refrigerator cooling or liquid nitrogen. Then, a temperature (operation temperature) of the SQUID varies depending on a cooling method to be used, and an optimum value of critical current of the SQUID also varies. For example, in the case that the SQUID is cooled via a rod cooled by liquid nitrogen, it is considered that the operation temperature is increased by 1 to 2 K in comparison with 77 K that is the operation temperature in the case that the SQUID is directly cooled by liquid nitrogen. In order to achieve a critical current of 20 to 100 μA at the operation temperature, it is required to design a critical current value to be larger at 77 K, for example, by forming a junction width to be wider in advance. Meanwhile, in a single HTS magnetic sensor, if it is capable to select a SQUID from a plurality of (integrated) SQUIDs that are different in characteristics and structures like different junction widths depending on the cooling method (operation temperature), the HTS magnetic sensor can be used without depending on/considering the cooling method (operation temperature). This is advantageous and useful.
As described above, if it is capable to select and use a desired SQUID from a plurality of SQUIDs within a single HTS magnetic sensor, a yield of the HTS magnetic sensor can be increased, a high sensitivity and wide dynamic rage are both obtained, and the HTS magnetic sensor can be used without depending on/considering a cooling method (operation temperature). This is advantageous and useful.
It is, therefore, an object of the present invention to provide a HTS magnetic sensor which has a plurality of SQUIDs and capable of selecting and using a desired SQUID from the plurality of SQUIDs, and a fabrication method of the HTS magnetic sensor.